1. Field of the Invention
This invention relates to a high strength spring steel useful in cars, aircraft, various industrial machines, etc.
2. Description of the Prior Art
In recent years, weight reduction has been strongly demanded in cars for lowering the cost of fuel. The same demand has also been growing in various structural parts or members including suspension devices. One possible approach for the reduction of weight of suspension devices is to increase the designed stress of suspension springs. In other words, strengthening the springs is effective as a weight-reducing measure. Currently, Si-Mn type steel, designated SUP 7, and Si-Cr type steel, designated SUP 12, are mainly used as steel stock for suspension springs. In order to increase the designed stress of these known spring steels, it is necessary to strengthen them. In general, the strength of steel materials is closely correlated with their hardness and strengthening means increasing the hardness. However, there is a problem that when the hardness of the spring steels is increased, the toughness (Charpy impact values, etc.) is also reduced. More specifically, a reduction in toughness is unavoidable in obtaining a hardness higher than that may be achieved in spring steels in current use. Therefore, when the hardness is increased for the purpose of improving the strength, the toughness must also be higher than that of currently available steels to ensure a sufficient reliability.